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AS and A Level: Waves & Cosmology
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- 1 When a source of waves is moving relative to an observer (either towards or away) the received waves have a different wavelength to the wavelength transmitted. This is known as the Doppler Effect and we can use it to calculate the speed of a galaxy relative to Earth.
- 2 Almost all galaxies show redshift, meaning that the wavelength received on Earth is longer than it was when transmitted. It’s called redshift because the wavelength received has moved towards tor even beyond the red end of the spectrum . Redshift implies that the galaxy is moving away from Earth.
- 3 Blueshift can be observed from ‘nearby’ stars and galaxies.
- 1 Using redshift data from a number of galaxies, Hubble plotted a graph of recession velocity, v, against distance to the galaxy, d. This graph continues to be updated and it shows that v = Hod which is known as Hubble’s law. This means that the speed of recession is directly proportional to the distance to the galaxy.
- 2 Ho is the Hubble constant and it has a value of about 70 km s-1 Mpc-1, which is equivalent to 2.3x10-18 s-1. 1/Ho= 4.4 x1017 s = 1.4 x 1010 years! This is the age of the universe, about 14 billion years.
- 3 We can also find an estimate for the size of the (visible) universe, assuming that the maximum expansion speed is the speed of light. Using Hubble law, c = Hod so d = c/Ho = 14 billion light years.
- 4 The uncertainty over the value of The Hubble constant is becoming smaller as measurements of distance to galaxies improve
- 5 Since redshift is seen in every direction, the conclusion is that the universe is expanding.
Fate of the universe
- 1 The fate of the universe is closely linked to CRITICAL DENSITY. This is a theoretical density that would have enough mass in the universe to keep the expansion of space slowing down forever. The critical density is given by o= 3H2/8 . The universe would be FLAT. An accurate value for H is important, if we want an accurate value for the critical density. Note: H2 means that the percentage uncertainty in H has to be doubled.
- 2 If the actual density is greater than the critical density, then the universe will stop expanding at some point and then collapse. The universe is then CLOSED. This outcome is known as the Big Crunch.
- 3 If the actual density is less than the critical density, there is not enough mass to stop the expansion and the universe will continue to expand forever. The universe is OPEN.
- 4 Determining the actual density is difficult because there seems to be dark matter which we cannot yet detect directly but which can be inferred by the gravitational effects it has. e.g the rotation of galaxies is not consistent with observable mass but with increased mass that may be explained by the presence of dark matter.
Therefore the load it can take will be 0g. I also predict that each time the rubber band is stretched it will be slightly longer then it was previously. This is because rubber is a polymer, which has a very tangled structure. It looks like this: - Every time it is stretched the structure becomes less tangled, thus making it longer. For this reason, I believe for each set of results the extension will increase slightly. Apparatus * Clamp and stand * Rubber band * 1g weights and holder * G clamp * Meter ruler * Mirror * Pin Safety I will ensure that the following things happen, to ensure the experiment is safe: - * Safety glasses are worn while the rubber band is being stretched.
- Word count: 1298
I took 7 results for both experiments. The preliminary tests, were to test the spring, and see how far it could stretch before exceeding its elastic limit. I loaded the spring up, with slotted masses. One at a time, until the spring broke. I measured the extension for each load, and plotted a graph from my results. I could see on my graph, that where the line was straight, the spring hadn't exceeded its elastic limit. Where the graph began to curve was where it had exceeded its limit.
- Word count: 1460
The elastic band will be catapulted off the end of another meter stick, in front of the ones that are taped to the wall. The band will be flung off the end of a meter stick, because the extension of the band can be measured easily using the units along the side of the meter rule. The height reached at each extension of the band will be recorded in a table of results along with a row of predicted heights at which the band may reach with each extension.
- Word count: 1585
To find out if Hooke’s Law can be proved with a steal spring and to see what would happen if the spring is stretched beyond the elastic limit.
To make the tests fair I will use the same spring and set of weights each time. Also I will add the weight proportionally in 1 Newton (10kg) each time. Therefore the change in weight will remain the same. My prediction is that Hooke's Law can be proved through a steel spring, as if the weight on the spring will increase so will the extension. The apparatus needed to do this experiment are a steel spring, a metre ruler, a clamp and weights. Hooke's Law found that extension is proportional to the downward force acting on the spring.
- Word count: 1372
Fair test I will keep this experiment fair by using the same spring each time to ensure the constant "k" stays the same. I will be pulling the spring down by the same amount each time and I will be using the same ruler to take all measurements. Range and Repeats I will be taking 10 readings ranging from 100g to 1000g I intend to repeat the experiment twice carrying it out 3 times in all. This way I can obtain more accurate and reliable results.
- Word count: 1047